Light-emitting diodes (LEDs) are a promising technology for flat-panel displays and area illumination lamps and backlights. Applications of LED devices include active-matrix image displays, passive-matrix image displays, and area-lighting devices such as, for example, selective desktop lighting. Irrespective of the particular LED device configuration tailored to these broad fields of applications, all LEDs function on the same general principles. An electroluminescent (EL) medium structure is sandwiched between two electrodes. At least one of the electrodes is at least partially light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the LED is said to be forward-biased. Positive charge carriers (holes) are injected from the anode into the EL medium structure, and negative charge carriers (electrons) are injected from the cathode. Such charge carrier injection causes current flow from the electrodes through the EL medium structure. Recombination of holes and electrons within a zone of the EL medium structure results in emission of light from this zone that is, appropriately, called the light-emitting zone or interface. The EL medium structure can be formed of a stack of sublayers that can include small molecule organic layers, polymer layers, or inorganic layers. Additional charge-control layers, for example hole-injection, electron-injection, hole-blocking, electron-blocking, hole-transport, electron-transport, and contact layers are known, may be employed for both organic and inorganic applications. Such layers and sublayers are known and understood by those skilled in the LED art.
Full-color LED devices may employ a variety of materials to emit different colors of light. In this arrangement, the LED device is patterned with different sets of materials, each set of materials associated with a particular color of light emitted. Each pixel in an active-matrix, full-color LED device typically employs each set of materials, for example to form a red, green, and blue sub-pixel. In an alternative arrangement, a single set of materials emitting broadband light may be deposited in continuous layers with arrays of differently colored filters employed to create a full-color LED device. In addition, black-matrix materials may be employed between the color filters in non-emissive areas of the LED device to absorb ambient light and thereby improve the contrast of the LED device. Such color filter and black-matrix materials are known in the art and are employed, for example, in the LCD industry. The contrast improvement possible by providing a black-matrix material between light-emitting areas of the LED device is limited by the relative size of the light-emitting areas and the areas between the light-emitting areas, i.e. the fill factor of the LED device.
The emitted light is directed towards an observer, or towards an object to be illuminated, through the light transmissive electrode. If the light transmissive electrode is between the substrate and the light-missive elements of the LED device, the device is called a bottom-emitting LED device. Conversely, if the light-transmissive electrode is not between the substrate and the light emissive elements, the device is referred to as a top-emitting LED device. The present invention is primarily directed to a top-emitting LED device.
In top-emitting LED devices, light is emitted through an upper electrode or top electrode, typically but not necessarily the cathode, which has to be sufficiently light transmissive, while the lower electrode(s) or bottom electrode(s), typically but not necessarily the anode, can be made of relatively thick and electrically conductive metal compositions which can be optically opaque. Because light is emitted through an electrode, it is important that the electrode through which light is emitted be sufficiently light transmissive to avoid absorbing the emitted light. Typical prior-art materials proposed for such electrodes include indium tin oxide (ITO) and very thin layers of metal, for example silver or aluminum or metal alloys including silver or aluminum. However, the current-carrying capacity of such electrodes is limited, thereby limiting the amount of power that can be supplied to the LED materials, and hence the amount of light that can be emitted from the light-emissive layers.
Referring to FIG. 11, a top-emitting LED device as suggested by the prior art is illustrated having a substrate 10 (reflective, transparent, or opaque). Over the substrate 10, a semiconducting layer is formed providing thin-film electronic components 30 for driving an LED. An interlayer insulating and planarizing layer 32 is formed over the thin-film electronic components 30 and a patterned reflective electrode 12 defining LED light-emissive elements is formed over the insulating layer 32. An inter-pixel insulating film 34 separates the elements of the patterned reflective electrode 12. One or more first layers 14 of light-emissive materials and any desired charge-control materials, are formed over the patterned reflective electrode 12. A transparent second electrode 16 is formed over the one or more first layers 14 of organic material to form an LED 11. A gap 19 separates the transparent second electrode 16 from an encapsulating cover 20. The encapsulating cover 20 is transparent and may be coated directly over the transparent electrode 16 so that no gap 19 exists. In some prior-art embodiments, the transparent electrode 12 may instead be at least partially transparent and/or light absorbing. Because suitable transparent conductors, for example ITO, have a limited conductivity, the current that may be passed through the organic layers 14 is limited and the uniformity of the light-emitting areas in an OLED device may be adversely affected by differences in current passed through various portions of the transparent conductor 16.
As taught in issued U.S. Pat. No. 6,812,637 entitled “OLED Display with Auxiliary Electrode” by Cok, an auxiliary electrode 70 may be provided between the light-emitting areas 24 of the LED to improve the conductivity of the transparent electrode 16 and enhance the current distribution to the LED. For example, a thick, patterned layer of aluminum or silver or other metals or metal alloys may be employed. However, the formation of the auxiliary electrode 70 is problematic. Sputtering through a shadow mask is difficult for large substrates due to thermal expansion and alignment problems of the shadow mask. Likewise, evaporative deposition of conductive materials such as metals requires high temperatures and suffers from the same mask problems. High temperatures may also destroy any temperature-sensitive underlying layers. The use of photolithography to pattern an auxiliary electrode may compromise the integrity of underlying layers, particularly for organic devices.
Co-pending, commonly assigned U.S. Ser. No. 11/179,186 filed 12 Jul. 2006 describes an organic light-emitting diode (OLED) device having an auxiliary electrode grid located above a transparent second electrode, providing spacing between the transparent second electrode and a cover. Co-pending, commonly assigned U.S. Ser. No. 11/253,909 filed 18 Aug. 2005 describes an organic light-emitting diode (OLED) device having an electrode with reflective and transparent portions in the light emissive area, the transparent portion being a relatively lower electrically conductive portion so that light emitted by the light-emitting organic layer passes through the transparent portion and the reflective portion being a relatively higher electrically conductive portion for reflecting emitted light. However, these disclosures do not address the robustness of any manufacturing process required to form patterned conductors in electrical contact with a transparent electrode and may suffer from the problems cited above. These disclosures are incorporated in their entirety by reference.
It is known in the prior art to form conductive traces using nanoparticles comprising, for example silver. The synthesis of such metallic nano-crystals is known. For example, U.S. Pat. No. 6,645,444 entitled, “Metal nanocrystals and synthesis thereof” describes a process for forming metal nanocrystals that involves complexing a metal ion and an organic ligand in a solvent and introducing a reducing agent to reduce a plurality of metal ions to form the metal nanocrystals associated with the organic ligand. The nanocrystals are optionally doped or alloyed with other metals.
US 2006/073667 entitled, “Stabilized silver nanoparticles and their use” describes a process comprising: reacting a silver compound with a reducing agent comprising a hydrazine compound in the presence of a thermally removable stabilizer in a reaction mixture comprising the silver compound, the reducing agent, the stabilizer, and an optional solvent, to form a plurality of silver-containing nanoparticles with molecules of the stabilizer on the surface of the silver-containing nanoparticles. The composition may be heated to form an electrically conductive layer comprising silver that may be employed in an electronic device. This disclosure describes sintering nanoparticles at a temperature of 120 C. However, applicants have demonstrated that the nanoparticles may have a relatively low conductivity when sintered at these temperatures. Applicants have determined through experimentation that compositions employing nanoparticles of 40 nm diameters are typically heated to 200 degrees C. or even 250 degrees C. to form adequately electrically conductive layers. Although lower temperatures may be employed with smaller-diameter particles, the sintering time tends to be longer. Moreover, the deposition and removal of any uncured composition may be difficult in the presence of any other environmentally sensitive materials. US 2006/073667 is hereby incorporated in its entirety by reference.
US 2006/0057502 entitled, “Method of forming a conductive wiring pattern by laser irradiation and a conductive wiring pattern” describes fine wirings made by a method having the steps of painting a board with a metal dispersion colloid including metal nanoparticles of 0.5 nm-200 nm diameters (preferably 1-30 nm), drying the metal dispersion colloid into a metal-suspension film, irradiating the metal-suspension film with a laser beam of 300 nm-550 nm wavelengths, depicting arbitrary patterns on the film with the laser beam, aggregating metal nanoparticles into larger conductive grains, washing the laser-irradiated film, eliminating unirradiated metal nanoparticles, and forming metallic wiring patterns built by the conductive grains on the board thus enabling an inexpensive apparatus to form fine arbitrary wiring patterns on boards without expensive photomasks, resists, exposure apparatus and etching apparatus. The method can make wirings also on plastic boards or low-melting-point glass boards that have poor resistance against heat and chemicals. US20060003262 similarly discloses a method of forming a pattern of electrical conductors on a substrate, wherein metal nanoparticles may be mixed with a light absorbing dye, and the mixture is then coated on the substrate. The pattern is formed on the coated substrate with laser light, and unannealed material is removed from the substrate. The disclosures of US 2006/0057502 and US 2006/003262 are hereby incorporated by reference herein in their entirety. Removing unannealed material can damage underlying sensitive materials in light-emissive areas. Commercial, patterned deposition methods, such as inkjet devices, may not provide the resolution necessary to avoid occluding light emission from a top-emitter LED device.
There is a need, therefore, for an improved method for providing increased conductivity to the transparent electrode of a top-emitting LED device that is scalable to large sizes, avoids heating materials in emissive locations to high temperatures, and avoids the use of chemical processes.